Goodwill and other intangible assets - Intangible Assets (Details) - Core Deposits - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Gross core deposit intangibles balance, beginning of period					$ 8,196	$ 8,196	$ 529	$ 529
Accumulated amortization, beginning of period					$ (718)	$ (513)	$ (27)	$ 0
Finite-lived Intangible Assets [Roll Forward]
Core deposit intangible, net, beginning of period	$ 7,478	$ 502	$ 7,683	$ 529
Established through acquisitions	0	7,667	0	7,667
CDI current period amortization	(205)	(77)	(410)	(104)
Total core deposit intangible, end of period	$ 7,273	$ 8,092	$ 7,273	$ 8,092